Grinding apparatus

ABSTRACT

A grinding apparatus which is so constructed that three workholders are rotatably mounted on a turn table in equally spaced relation peripherally of said turn table to be intermittently shifted from one to another of three positions consisting of an inoperative position and a first and second working positions. Two grinding mechanisms are arranged exterior of said turn table adjacent said first and second working positions respectively. The table is operative in such a manner that a workpiece mounted on one of said workholders in said inoperative position is carried thereon to said first working position where it is roughly ground and then to said second working position where it is subjected to a finishing grinding and then back to said inoperative position to be removed from the apparatus, the intermittent rotation of said turn table being controlled by a detecting device which detects the amount of the workpiece ground by way of a master which is ground concurrently with said workpiece.

United States Patent Yamanaka 1 May 30, 1972 [54] GRINDING APPARATUS I Primary Examiner-Harold D. Whitehead [72] Inventor. Sadajiro Yamanaka, Chigasakn Japan Attorney stevens, Davis Miner & Mosher [73] Assignee: Yugen Kaisha Yamanaka Seisakusho,

Chigasaki-shi, Kanafawa-ken, Japan [5 7] ABSTRACT [22] Filed: Sept. 28, 1970 A grinding apparatus which is so constructed that three workholders are rotatably mounted on a turn table in equally [21] Appl' 75842 spaced relation peripherally of said turn table to be intermittently shifted from one to another of three positions consisting [30] Foreign Application Priority Data of an inoperative position and a first and second working positions. Two grinding mechanisms are arranged exterior of said Dec. 18, 1969 Japan ..44/101837 mm table adjacent said first and second working positions respectively. The table is operative in such a manner that a US. Cl ..slll34il1g workpiece mounted on one of said workholders in said operative position is carried thereon to said first working posi- [58] Field of Search ..51/134, 165.77, 165.78 on where it is g y ground and then to Said second work ing position where it is subjected to a finishing grinding and [56] References cued then back to said inoperative position to be removed from the UNITED STATES PATENTS apparatus, the intermittent rotation of said turn table being controlled by a detecting device which detects the amount of Wallace ..5 X the workpiece ground way ofa master is ground con- 2,656,653 10/1953 Gardner ..5 l/165.78 X currently with said workpiece 3,305,978 2/1967 Lumsden .....5l/l31 X 2,940,224 6/1960 Madrich ..51/131 7 Claims, 15 Drawing Figures IIC Patented May 30, 1972 3,665,648

9 Sheets-Sheet 1 SADAJIRO YAMANAKA INVENTOR I BY flan/01 ATTORN Y5 Patented May 30, 1972 3,665,648

9 Sheets-Sheet 2 Patented May 30, 1972 3,665,648

9 Sheets-Sheet F5 Patented May 30, 1972 9 Sheets-Sheet 4 Patented May 30, 1972 3,665,648

9 Sheets-Sheet 5 Patented May 30, 1972 9 Sheets-Sheet 6 Patented May 30, 1972 3,665,648

9 Sheets-Sheet 7 FIG.I|

Patented May 30, 1972 3,665,648

9 Sheets-Sheet 6 F I G. I3

Patented May 30, 1972 9 Shee ts-Shoet (J wumDOm KUZOQ wm Im n GRINDING APPARATUS The present invention relates to a grinding apparatus for a workpiece and has for its object the provision of such a grinding apparatus which is capable of grinding a workpiece more efficiently and finishing the same with higher accuracy than conventional apparatus.

An embodiment of the present invention will be described hereinafter with reference to the accompanying drawings, in which:

FIG. 1 is a plan view showing the overall arrangement of the grinding apparatus according to the present invention;

FIG. 2 is a perspective view of a portion of the apparatus;

FIG. 3 is a cross-sectional view of a turn table driving mechanism;

FIG. 4 is a plan view briefly showing the arrangement of a slip ring for controlling the rotation of a grinder;

FIG. 5 is a plan view showing the power transmitting arrangement of the turn table driving mechanism;

FIG. 6 is a cross-sectional view showing a master driving mechanism of a measuring device;

FIG. 7 is a plan view showing the arrangement the turn table, workholder and master driving mechanism;

FIG. 8 is a cross-sectional view of the measuring device;

FIG. 9 is a plan view of the measuring device looking in the direction of the line IXIX of FIG. 8;

FIGS. 10 and 1 l are a side view and a plan view respectively, showing in detail a portion of the measuring device;

FIG. 12 is a cross-sectional view of the entire grinding mechanism;

FIG. 13 is a cross-sectional view showing a driving device for the grinding mechanism; and

FIG. 14 is a cross-sectional view taken on the line XlV XIV of FIG. 13.

FIG. 15 is a schematic wiring diagram of the control circuit of the measuring device.

OUTLINE OF THE APPARATUS The apparatus of the instant invention comprises a turn table assembly T, a left hand grinding mechanism LG, a right hand grinding mechanism RG and a coolant supply mechanism C (FIG. 1). The turn table assembly T includes a turn table 10 rotatably arranged in a horizontal plane and three workholders 11 arranged in substantially the same direction as the turn table in equally spaced relation and rotatable relative to said turn table. The workholders 11 are made of a magnetic material and hold a workpiece (not shown) thereon under the magnetic force thereof. Two grinding mechanisms LR,RG are arranged in positions corresponding to two of the three workholders 11, i.e. positions 11a, 11b. Each of the grinding mechanisms has a cup-shaped grinding wheel 12 supported in opposed relation to the upper surface of the workholder 11 (FIG. 2) and the grinding wheel is rotatably driven in contact with the workpiece, retained on the workholder 11 to grind said workpiece. The grinders 11 are rotatably driven when they are in the positions corresponding to the grinding mechanisms LG, RG, or in the working positions, and are held inoperative in the other position, i.e. the position indicated by 11c in FIG. 1. The workpiece is mounted on the workholder while in the inoperative position He and then the turn table 10 is rotated in a clockwise direction as viewed in FIG. 1 to place that workholder in a first working position indicated by 11a, where it is driven. At the same time, the grinding wheel 12 of the grinding mechanism LG is lowered to grind the workpiece. In the first working position, the workpiece is ground roughly. Then, the turn table is further rotated in a clockwise direction and the workpiece is brought to a second working position indicated by 11b, where it is subjected to a finishing grinding. Thereafter, the turn table 10 is further rotated in a clockwise direction and the grinder is shifted to the inoperative position indicated by 11c, where the finished workpiece is demounted from the workholder and a new workpiece is mounted on the workholder ll.

In the apparatus of this invention, a measuring device or master 82 is provided adjacent each workholder 11. This master 82 is a cylindrical body made of such a material, for example, as bakelite, and is driven in association with the workholder. In the grinding operation, the grinding wheel 12 in contact with the workpiece is also held in contact with the master 82 and grinds said it at the same rate as the workpiece. A detector element 109a of an electric micrometer 109 is held in contact with the upper edge of the master 82 to detect the point when the master has been ground by a predetermined amount and thereby to control the interruption of the grinding operation and shifting of the workholder 11 to the next grind ing step. Cooling water is supplied to the grinding portion from the cooling device C throughout the grinding operation.

Turn Table Assembly The turn table 10 of the turn table assembly T is secured to a main vertical rotary shaft 15 at its center by means of a plurality of bolts 16 (FIG. 3). A worm wheel 16 is fixedly mounted on the lower end of the main rotary shaft 15 as by key 17, and a worm 18 is in meshing engagement with the worm wheel 16. The main rotary shaft 15 is driven by the rotation of the worm 18. A hollow cylindrical rotary sleeve 19 is provided concentrically around the lower portion of the main rotary shaft 15, with the upper and lower ends thereof rotatably mounted on said main rotary shaft through bearings 20, 21 respectively. The exterior of the rotary sleeve 19 is surrounded by a hollow cylindrical fixed sleeve 22 which is secured to a frame F. The rotary sleeve 19 is rotatably supported on the fixed sleeve by bearings 23, 24. A gear 25 is fixed to the upper end of the rotary sleeve 19 by a pin 26 and a bearing 27 is interposed between the underside of the gear 25 and the upper edge of the fixed sleeve 22. Further, a bearing 28 is interposed between the lower end portion of the main rotary shaft 15 and the fixed sleeve 22. A worm wheel 19a is formed at the lower end portion of the rotary sleeve 19, which is in meshing engagement with a worm 29. The rotary sleeve 19 is rotated by the rotation of the worm 29.

A work spindle 30 to drive the workholder 11 is arranged in parallel to the main rotary shaft 15 and spaced therefrom in the radial direction of the turn table 10. A disc-shaped supporting plate 31 is fixed to the top end of the spindle 30 as by bolts 32 and rotatably supported by the upper surface of a spindle case 33, fixed to the turn table 10, and an annular projection 33a formed thereon. The spindle case 33 has a cylindrical portion 33b extending downwardly so as to surround the entire length of the spindle 30. The cylindrical portion 33b is formed therein with an opening 330 on the side facing the main rotary shaft,,through which the gear 25 extends into the spindle case 33. On the spindle 30 is rotatably mounted by a bearing 35 a gear 34 which meshes with the gear 25. A spline part 36 is fixed to the underside of the gear 34 by means of a bolt 37 and a clutch element 38 is in spline engagement with said spline part 36. A clutch element 39 co-operating with the clutch element 38 is fixed to the spindle 30 by means of a key 40 and electromagnetic coil 41 is disposed within the clutch element 39. The coil 41 is secured by means of a bolt 44 to an annular body 43 which is rotatably mounted on the spindle 30 through a bearing 42. The annular body 43 is non-rotatably fixed to the spindle case 33 by means of a stop pin 45. Therefore, when the coil 41 is excited, the clutch element 38 is attracted towards the clutch element 39 and brought into engagement therewith, so that the drive of the main rotary shaft is transmitted to the spindle 30 through the gear 34 and the clutch elements 38, 39. As stated previously, the workholder 11 is made of a magnetic material and hence capable of holding the workpiece magnetically.

As shown in FIG. 3, an annular body 46 made of an insulating material is fixed to the outer surface of the fixed sleeve 22 as by a pin 47, and a ring 48, made of an electrically conductive material, is fitted around said annular body 46. The ring 48, as shown in FIG. 4 is split into a portion 48a which extends along about three-fourths of the entire circumference of the annular body 46 and a position 48b which extends along the remainder of said circumference. The portion 480 is connected to a power source through a conductor not shown. A brush 50 is fixed to the spindle case 33 by means of a bracket 49 as shown in FIG. 4, which will slide on the ring 48 in contact therewith. The brush 50 is connected to the coil 41 through a suitable conductor (not shown). The arrangement is such that, when the brush 50 is in contact with the portion 48a of the ring 48, the coil 41 is excited to drive the spindle 30, whereas when the brush 50 is in contact with the portion 48b of said ring, the current supply to the coil 41 is interrupted and hence the spindle 30 stops rotating, The point where the brush 50 is brought into contact with the portion 48b of the ring 48, corresponds to the inoperative position 1 1c of the workholder mentioned previously.

A mechanism for transmitting rotation to the worms 18 and 29 is shown in FIG. 5. The shaft 51 of the worm 18 is horizontally and rotatably supported by a pair of bearings 52, 53 and has a pulley 54 connected to one end thereof. A motor 55, provided outside the column of the grinding mechanism RG to be described later, drives a pulley 57 through a reduction gear 56 and the drive of the motor 55 is transmitted to the worm shaft 51 through a belt 58 engaged around the pulley 57 and the aforesaid pulley 54. The shaft 59 of the worm 29 is rotatably supported by a pair of bearings 60, 61 and has a pulley 62 connected to one end thereof. A motor 63, provided outside of the column of the grinding mechanism LG to be described later, drives a pulley 65 through a reduction gear 64 and the drive of the motor 63 is transmitted to the worm shaft 59 through a belt 66 engaged around the pulley 65 and the aforesaid pulley 62. The motor 55 is operated by an instruction from the measuring device 109 as will be described later, and set in motion when the workpiece has been ground to a predetermined dimension, to cause rotation of the turn table 10. An index means may be provided as required, to deter mine the position of the table 10.

Measuring Device A master 82 is provided adjacent each workholder 11. As shown in FIG. 6, its drive includes a hollow cylindrical drive shaft 69 having a gear 67 fixed on the lower end thereof by a key 68, and a center shaft 70 extending through the axial bore of said drive shaft 69. The center shaft 70 and the drive shaft 69 are combined by a key groove 70b and a key 71 slidably received in said key groove, in such a manner that they are longitudinally slidable relative to each other but rotate all together. As shown in FIG. 3, a gear 72 is fixedly mounted on the spindle 30 above the gear 34, and this gear 72' turns with the aforesaid gear 67 through an intermediate gear 73 (FIG. 7). Therefore, when the spindle 30 is driven, the drive shaft 69 and the center shaft 70 of the measuring device 13 are driven concurrently. Exterior of the drive shaft there is provided an inner sleeve 73 through a roller bearing 71 and a ball bearing 72, and an outer sleeve 74 is axially slidably provided around the inner sleeve 73. The sleeve 74 is formed with an inward flange 740 at the upper end thereof and a bearing 75 is interposed between said flange 74a and an annular flange 700 formed on the center shaft 70 adjacent the upper end thereof. A support disc 76 is connected to the upper end of the center shaft 70 by a key 77 and fixed thereto by means of a nut 78, for rotation therewith. A bearing 79 is interposed between the support disc 76 and the upper end of the outer sleeve 74. A master supporting member 80 is fixed on top of the support disc 76 by means of a bolt 81 and a cylindrical master 82 is fixed on top of the master supporting member 80 by a retainer ring 83 and a cap 84. The cap 84 is secured to the master supporting member 80 through threadable engagement between it and an externally threaded stud 80a provided centrally of the upper surface of the master supporting member 80. The master 82 is preferably made of such a material as bakelike. When the gear 67 is driven, the master supporting assembly constructed as described above is rotated, with the drive shaft 69, the center shaft 70, the support disc 76, the master supporting member 80, the master 82, the retainer ring 83 and the cap 84 as an integral unit.

The master supporting assembly described above is supported by axially slidably fitting the outer sleeve 74 in a supporting bracket 85 which is fixed to the frame F. The outer sleeve 74 is formed with a axial key groove 74b in the outer peripheral surface thereof, and a key 86 fixed to the bracket 85 is received in said key groove. Further, external threads are formed in the lower portion of the outer peripheral of the outer sleeve 74 and a gear 87 is mounted on said outer sleeve, with internal threads of the former in engagement with said external threads of the latter. Therefore, when the gear 87 is rotated, the outer sleeve 74 and hence the master 82 is elevated. A lock screw 88 is extended horizontally through the wall of the supporting bracket 85, with the inner end thereof engaging the external threads on the outer surface of the outer sleeve 74 to lock the vertical position of said outer sleeve. The gear 87 is rotatably supported by bearings 89 provided on the upper and lower surfaces thereof and can be rotated by a mechanism including a manual adjusting wheel 90. The manual adjusting wheel 90 is fixed on a vertical shaft 93 by means of a key 94, which vertical shaft 93 is supported by a supporting plate 91, fixed to the frame F, through a bearing 92. The vertical shaft 93 has a gear 95 fixedly mounted on the lower end thereof by a bolt 96, and the gear 95 is meshing with the gear 97 through an intermediate gear 97.

Detector Assembly The detector assembly is provided in combination with each one of the left and right hand grinding mechanisms LG, RG. As shown in FIG. 1, each detector assembly includes a vertical post 98 (FIG. 8) fixed to the frame F exterior of the turn table, and a cylindrical housing 101 is rotatably mounted on the vertical post 98 to surround the same, through bearings 99, 100. A supporting cylinder 102 is fixed to the upper end of the cylindrical housing 101 by a bolt 103, and an arm 105 is connected to the outer surface of said supporting cylinder 102 through a bracket 104. The post 98 extends through the supporting cylinder 102, and a lock screw 106 is mounted on a threaded portion 98a at the top end thereof. The lower surface of the lock screw 106 acts on the upper edge of the supporting cylinder 102 through a washer 107, to hold the housing 101 and the supporting cylinder 102 against rotation. A handle 108 is provided on the lock screw 106 and, by loosening the lock screw 106 by said handle, the housing 101 and the supporting cylinder 102 are rendered rotatable relative to the post 98.

The arm 105 has a detector such, for example, as an electric micrometer 109 fixed to one end thereof, and a detector operating bracket 110 is fixed to the detector 109 below the arm 105. The bracket 110 is formed with a flange extending downwardly from one end thereof and a horizontal shaft 112 is rotatably fitted to said flange. A lever 113 and a lift plate 114 are connected to one end and the central portion of the horizontal shaft 112 respectively. A tension spring 115 has its lower end anchored to one end of the lever 113, with the upper end thereof anchored to a rod 118 which is connected to a vertically movable head 1 16 of the grinding mechanism to be described later, through a bracket 117 in such a manner that the vertical position thereof may be adjusted. The free end of the lift plate 114 is located beneath the lower surface of a movable portion 109a of the micrometer 109. Thus, when the vertically movable head 1 16 moves upwardly, the free end of the lift plate 114 is elevated, pushing the movable portion 109a of the micrometer 109, so that the movement of the master 82 will not interfer with micrometer 109 during rotation of the turn table. The head 116 is lowered in the grinding operation and, therefore, the lift plate 114 is held away from the micrometer 109 as shown in FIG. 10. Reference numeral 119 designates a stop pin to define the lowered position of the lever 113 and hence of the lift plate 114. During the grinding operation, the tip end of a detecting needle of the micrometer rests on the upper surface of the master 82, to detect the amount of the master and, therefore, the amount of the workpiece which has been cut.

Grinding Mechanism The apparatus of the instant invention is providedwith two grinding mechanisms LG, RG but, since both grinding mechanisms are identical in construction, only one of them will be described herein with reference to FIGS. 12, 13 and 14.

The grinding mechanism includes a head member 121 vertically slidably mounted in a column 120 fixed to the frame F, and a nut member 123 is fixed on the head member 121 by a bracket 124. The nut member 123 is in threadable engagement with a threaded drive shaft 122 vertically rotatably extending through the column 120. It will, therefore, be seen that the head member 121 can be moved vertically by rotating the threaded drive shaft 122. The shaft 122 is driven at two different speeds, i.e., a high speed and a low speed, by two systems, i.e. a quick feed feed mechanism and a grinding feed mechanism, respectively. The quick feed mechanism consists of a worm wheel 126 which is rotatably mounted on the lower end of a shaft 122 and coupled therewith by an electromagnetic clutch 125, and a worm 128 which is mounted on the output shaft of a motor 127 and adapted to engage said worm wheel 126. The grinding feed mechanism includes a grinding feed motor 129 (FIG. 13). The output shaft of the motor 129 is connected to a shaft 132 through gears 130, 131. The shaft 132 has a worm 133 formed at one end thereof, which meshes with the worm wheel 135 on an intermediate shaft 134 (FIG. 14). Intermediate shaft 134 has a worm 136 which meshes with a worm wheel 138 on a shaft 137. The worm wheel 138 is rotatably mounted on the shaft 137 and coupled therewith by an electromagnetic clutch 139. On one end of the shaft 137 is fixedly mounted a bevel gear 140 which meshes with a bevel gear 141 fixedly mounted on the shaft 122. Thus, when the clutch 139 is excited, with the motor 129 in motion, the shaft 122 is driven at the low speed, that is, the grinding feed speed. Further, a handle 142 is provided at the outer end of the shaft 137. By rotating the handle 142, the head member can be manually moved up and down. Further, on the bracket 124 supporting the nut 123 a second nut 143 is provided which is in threadable engagement with the shaft 122, and a worm wheel 144 is formed on the outer periphery of said nut 143, which meshes with an adjusting worm 145. Thus, it will be understood that, by rotating the adjusting nut 145, the nut 143 is rotated and the back-lash between the first nut 123 and the threads of the shaft 122 can be eliminated by the slight rotation of said nut 143. A balance weight 149 is provided on the head member 121 by means of a wire 148 which is engaged around pulleys 146, 147 provided on the column 120.

A motor 150 is fixed on the upper surface of the head member 121 and a pulley 151 is mounted on the output shaft of said motor. The pulley 151 is operatively connected with a pulley 153 through a belt 152, which pulley 153 is fixed on the upper end of a rotary shaft 155 rotatably received in a cylindrical vertical bore formed in the head member 121. The cupshaped grinding wheel 12 is fixed to the lower end of the rotary shaft 155 by a suitable means. The grinding wheel 12 is positioned above the workholder l1 and the master 82 of the measuring device in the working position. Therefore, when the motor 150 is set in motion, the grinding wheel 12 is rotated at the grinding speed, whereby the workpiece on the workholder 11 can be ground.

Operation In operating the apparatus of the present invention, a workpiece is mounted on the workholder in an inoperative position 110, which is capable of holding the metallic workpiece thereon as it is made of a magnetic material. Here, the turn table is turned in a clockwise direction as viewed in FIG. 1 and the workholder 11, with the workpiece mounted thereon, is shifted to below the grinding wheel 12 of the grinding mechanism LG, that is, the first working position. Immediately before reaching the first working position the workholder 11 is caused to rotate by the mechanism described previously with reference to FIGS. 3 and 4. When the workholder 11 has reached the first working position, the rotation of the turn table 10 is stopped and the head member 121 of the grinding mechanism LG is lowered and the grinding wheel 12 is brought into contact with the workpiece to grind the same.

The head member 121 is lowered in two steps. Namely, the head member is lowered by the motor 127 at a relatively high speed until immediately before the grinding wheel 12 contacts the workpiece and then is lowered by the motor 129 at a relatively low speed as a result of the electromagnetic clutch being demagnetized and the electromagnetic clutch 139 being excited. As stated previously, the grinding wheel 12 grinds the workpiece and the master 82 of the measuring device concurrently. The amount of the master 82 ground is detected by the detecting device and, when the amount has reached to a predetermined value, the shaft 122 is driven in a reverse direction in response to an electric signal from the electric micrometer of the detecting device, to lift the head member 122. The control circuit to accomplish this reverse direction is shown in FIG. 15. When the master is sufiiciently ground, a microswitch in micrometer 109 closes, actuating a solenoid which in turn actuates a reversing switch feeding motor 127. After the head is lifted, the turn table 10 is rotated and the workholder 11 is shifted to the second working position, i.e. a position corresponding to the grinding mechanism RG, with the roughly ground workpiece thereon, where the workpiece is subjected to a finishing grinding.

The apparatus of the invention is particularly adapted for use in grinding a number of small-sized workpieces on the grinder. In a case where it is impossible to measure the amount of the workpiece ground by contacting the electric micrometer directly with the workpiece, this problem can be solved by providing a master separate from the workpiece. In addition, the grinding is effected in two stages, i.e. by a rough grinding operation and a finishing grinding operation, and these grinding operations can be carried out concurrently by two grinders. Yet further, in the process of the grinding operations, the finished workpiece can be removed from and a new workpiece can be mounted on the workholder which is located in an inoperative position. Thus, the grinding operation can be carried out in a highly efficient manner.

I claim:

1. A grinding apparatus comprising a frame, a turn table rotatably mounted on said frame, three disc-shaped workholders rotatably mounted on said turn table in equally spaced peripheral relation on said turn table, two grinding mechanisms arranged exterior of said turn table in spaced relation from said turn table, means for rotating said turn table intermittently in such a manner that one of said workholders may be held stationary in position corresponding to one of said grinding mechanisms and another one in a position corresponding to the other grinding mechanism, means for rotating each one of said workholders in a path of movement from the position corresponding to one of said grinding mechanisms to the position corresponding to the other grinding mechanism and holding it stationary in the other position, a master provided adjacent each of said workholders and operatively connected to said workholder to be rotated thereby, each of said grinding mechanisms including a grinding wheel arranged to grind a workpiece on the workholder and said master concurrently, and means for detecting the amount of said master ground by said grinding wheel and emitting a signal to stop the grinding operation.

2. The apparatus of claim 1 including magnetic means associated with said workholders to retain the workpiece thereon.

3. The apparatus of claim 1 in which the turn table is circular and the workholders follow an arcuate path in movement from one position to the next.

4. The apparatus of claim 3 in which said turn table is horizontal and said grinding mechanisms are located vertically above the turn table. a

5. The apparatus of claim 1 including means to move said grinding wheels in a rotary direction in a plane parallel to said workholders.

6. The apparatus of claim 5 in which the grinding wheels are cup-shaped.

7. The apparatus of claim 1 including a two speed means for moving the grinding mechanism toward the workpiece with a high speed for setting up operation and a low speed for grinding. 

1. A grinding apparatus comprising a frame, a turn table rotatably mounted on said frame, three disc-shaped workholders rotatably mounted on said turn table in equally spaced peripheral relation on said turn table, two grinding mechanisms arranged exterior of said turn table in spaced relation from said turn table, means for rotating said turn table intermittently in such a manner that one of said workholders may be held stationary in position corresponding to one of said grinding mechanisms and another one in a position corresponding to the other grinding mechanism, means for rotating each one of said workholders in a path of movement from the position corresponding to one of said grinding mechanisms to the position corresponding to the other grinding mechanism and holding it stationary in the other position, a master provided adjacent each of said workholders and operatively connected to said workholder to be rotated thereby, each of said grinding mechanisms including a grinding wheel arranged to grind a workpiece on the workholder and said master concurrently, and means for detecting the amount of said master ground by said grinding wheel and emitting a signal to stop the grinding operation.
 2. The apparatus of claim 1 including magnetic means associated with said workholders to retain the workpiece thereon.
 3. The apparatus of claim 1 in which the turn table is circular and the workholders follow an arcuate path in movement from one position to the next.
 4. The apparatus of claim 3 in which said turn table is horizontal and said grinding mechanisms are located vertically above the turn table.
 5. The apparatus of claim 1 including means to move said grinding wheels in a rotary direction in a plane parallel to said workholders.
 6. The apparatus of claim 5 in which the grinding wheels are cup-shaped.
 7. The apparatus of claim 1 including a two speed means for moving the grinding mechanism toward the workpiece with a high speed for setting up operation and a low speed for grinding. 